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datetime.strptime without a year fails on Feb 29 #70647
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SriramRajagopalan commented
on Feb 29, 2016 SriramRajagopalanmannequinMannequinAuthor$ python Python 3.5.1 (default, Dec 7 2015, 12:58:09) [GCC 5.2.0] on linux Type "help", "copyright", "credits" or "license" for more information. >>> >>> >>> >>> import time >>> >>> time.strptime("Feb 29", "%b %d") time.struct_time(tm_year=1900, tm_mon=2, tm_mday=29, tm_hour=0, tm_min=0, tm_sec=0, tm_wday=0, tm_yday=60, tm_isdst=-1) >>> >>> >>> import datetime >>> >>> datetime.datetime.strptime("Feb 29", "%b %d") Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/usr/lib/python3.5/_strptime.py", line 511, in _strptime_datetime return cls(*args) ValueError: day is out of range for monthThe same issue is seen in all versions of Python
Python's time.strptime() behavior is consistent with that of glibc 2.19:
======= strptime_c.c =======
#define _XOPEN_SOURCE #include <stdio.h> #include <stdlib.h> #include <string.h> #include <time.h> int main(void) { struct tm tm; char buf[255]; memset(&tm, 0, sizeof(struct tm)); strptime("Feb 29", "%b %d", &tm); strftime(buf, sizeof(buf), "%d %b %Y %H:%M", &tm); puts(buf); exit(EXIT_SUCCESS); }
=======
$ gcc strptime_c.c $ ./a.out 29 Feb 1900 00:00
I'm not saying that the behavior is a good API, but given the unfortunate API at hand, parsing a date without specifying what year it is using strptime is a bad idea.
This is not no more bug than
>>> from datetime import * >>> datetime.strptime('0228', '%m%d') datetime.datetime(1900, 2, 28, 0, 0)
Naturally, as long as datetime.strptime('0228', '%m%d') is the same as datetime.strptime('19000228', '%Y%m%d'), datetime.strptime('0229', '%m%d') should raise a ValueError as long as datetime.strptime('19000229', '%Y%m%d') does.
The only improvement, I can think of in this situation is to point the user to time.strptime() in the error message. The time.strptime method works just fine in the recent versions (see bpo-14157.)
>>> time.strptime('0229', '%m%d')[1:3] (2, 29)
time.strptime() is "working" (not raising an exception) as it appears not to validate the day of the month when a year is not specified, yet the return value from either of these APIs is a date which has no concept of an ambiguous year.
## Via the admittedly old Python 2.7.6 from Ubuntu 14.04: ## # 1900 was not a leap year as it is not divisible by 400. >>> time.strptime("1900 Feb 29", "%Y %b %d") ValueError: day is out of range for month >>> time.strptime("Feb 29", "%b %d") time.struct_time(tm_year=1900, tm_mon=2, tm_mday=29, tm_hour=0, tm_min=0, tm_sec=0, tm_wday=0, tm_yday=60, tm_isdst=-1)
So what should the validation behavior be?
>>> datetime.datetime.strptime("Feb 29", "%b %d") ValueError: day is out of range for month >>> datetime.datetime.strptime("2016 Feb 29", "%Y %b %d") datetime.datetime(2016, 2, 29, 0, 0) >>> datetime.datetime.strptime("1900 Feb 29", "%Y %b %d") ValueError: day is out of range for month >>> datetime.datetime(year=1900, month=2, day=29) ValueError: day is out of range for month
datetime objects cannot be constructed with the invalid date (as the time.strptime return value allows).
Changing the API to assume the current year or a +/- 6 months from "now" when no year is parsed is likely to break existing code.
nickzoic commented
on Mar 2, 2020 nickzoicmannequinMannequinI suspect this is going to come up about this time of every leap year :-/
The workaround is prepending "%Y " to the pattern and eg: "2020 " to the date string, but that's not very nice.
Would adding a kwarg "default_year" be an acceptable solution?
I can't think of any other situation other than leap years when this is going to come up. If both "default_year" and "%Y" are present throw an exception (maybe therefore just call the kwarg "year")In the weird case where you want to do date maths involving the month as well, you can always use a safe choice like "default_year=2020" and then fix the year up afterwards:
dt = datetime.strptime(date_str, "%b %d", default_year=2020) dt = dt.replace(year=2021 if dt.month > 6 else 2022)Reacted by Gregory P. Smith and ryssonI don't think adding a default_year parameter is the right solution here.
The actual problem is that
time.strptime, and by extensiondatetime.strptimehas a strange and confusing interface. What should happen is either thatyearis set to None or some other marker of a missing value or datetime.strptime should raise an exception when it's being asked to construct something that does not contain a year.Since there is no concept of a partial datetime, I think our best option would be to throw an exception, except that this has been baked into the library for ages and would start to throw exceptions even when the person has correctly handled the Feb 29th case.
I think one possible "solution" to this would be to raise a warning any time someone tries to use
datetime.strptimewithout requesting a year to warn them that the thing they're doing only exists for backwards compatibility reasons. We could possibly eventually make that an exception, but I'm not sure it's totally worth a break in backwards compatibility when a warning should put people on notice.nickzoic commented
on Mar 2, 2020 nickzoicmannequinMannequinNot disagreeing with you that "%b %d" timestamps with no "%Y" are excerable, but they're fairly common in the *nix world unfortunately.
People need to parse them, and the simple and obvious way to do this breaks every four years.
I like the idea of having a warning for not including %Y *and* not setting a default_year kwarg though.
47 remaining items
I was writing this up today, however, first we should decide when exactly we are going to do whatever it is we are going to do (i.e. always raise or change the default year).
The initial deprecation was merged into 3.13, so 3.15 (which is noted in the docs and error message) is too early to do anything.
%e's addition was backported to 3.13.5 (ea25f4a) but only documented in 3.15, so I propose we schedule the change to both in 3.18. This way%d's deprecation meets the PEP 387 guideline, and%egets three years, but since it was never documented, I think it is unlikely someone is using it in such a way.Reacted by Petr Viktorin- added a commit that references this issue
on Apr 15, 2026 remaining todo: in 3.17 turn %e usage without a year into an error.
remaining todo: in 3.17 turn %e usage without a year into an error.
Let's make that a new issue, we won't be needing it for a while, as such, closing.
- moved this from In Progress to Done in Date and time issues 🕰️
on Apr 15, 2026 - added a commit that references this issue
on Apr 25, 2026 - added a commit that references this issue
on May 26, 2026 - added a commit that references this issue
on Jul 27, 2026 - added a commit that references this issue
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%einto an error in 3.17.Linked PRs
%d(and deprecate for%e) without year instrptime()#144570